/Engine/Source/Runtime/Slate/Public/Widgets/Layout/SSplitter.h
Given the index of the dragged handle and the children, find a child above/left_of of the dragged handle that can be resized.
INDEX_NONE if no such child can be found.
static int32 FindResizeableSlotBeforeHandle ( int32 DraggedHandle, const [TPanelChildren](API\Runtime\SlateCore\Layout\TPanelChildren)< [FSlot](API\Runtime\Slate\Widgets\Layout\SSplitter\FSlot) > & Children )